This invention relates to fixing rolls for electrophotography, and more particularly it is concerned with a roll for heating and fixing a toner image on a support sheet in electrophotography.
The quickest and most positive method for fixing a toner image formed on a support sheet as by transfer printing consists in directly heating the toner image with a heating roll to cause the same to adhere by fusion or melt adhesion to the copy sheet. It has hitherto been in practice to bring the toner image into direct contact wih a heated surface, such as the periphery of a heating roll, to fix the toner image.
This method has, however, a disadvantage in that what is referred to as an offset phenomenon tends to occur during the process. More specifically, when a toner image is heated and the toner particles become tacky, some portions of the toner image supported by a support sheet may adhere to the surface of the heating roll and may be transferred to the next following sheet. Then, some portions of the toner image on the second sheet may adhere to the heating roll and may be transferred to the third sheet. This cycle may be repeated till a desired number of copies have been produced. The transfer of portions of the toner image on the preceding support sheet to the next following support sheet is the aforementioned offset phenomenon.
The offset phenomenon is not desirable and should be precluded in fixing toner images on support sheets. In order to preclude the offset phenomenon, it is required that the peripheral surface of the heating roll should be finished such that the surface is as smooth as glass. However, conventional heating rolls are constructed such that it is impossible to impart glass-like smoothness to their peripheral surfaces by grinding.
More specifically, when non-tacky rolls are made, their peripheral surfaces are ground. Thus, the surfaces are matted, and it is impossible to render the surfaces of rolls as smooth as glass. Toner images tend to adhere to such surfaces even if they are non-tacky.
On the other hand, toner image fixing rolls are preferably resilient and heat conductive. The reason why such rollers are preferably resilient will be explained in some detail immediately below in connection with FIGS. 1 and 2.
FIG. 1, which is slightly exaggerated to enable essential points to be readily understood, shows a heating and fixing device comprising a heating and fixing roll 2 adapted to be brought directly into engagement with a toner image P formed on a support sheet 1, and a pressing roll 3 disposed over heating and fixing roll 2 and adapted to bear against the same. Heating and fixing roll 2 and pressing roll 3 are driven by drive means (not shown) to rotate in the directions of arrows a and b respectively.
The support sheet 1 carrying the toner image P thereon is fed between the two rollers 2 and 3 so as to heat and fix the image P on support sheet 1. If heating and fixing roll 2 had no resilience, support sheet 1 would tend to get stuck on the peripheral surface of heating and fixing roll 2 and move with roll 2 in the direction of an arrow C even if means is provided for precluding the offset phenomenon, because pressing roll 3 has resilience and increases the amount of contact between the two rollers. Thus, copy sheet jam would ensue. In order to preclude copy sheet jam, it is desirable that heating and fixing roll 2 should have resilience.
If heating and fixing roll 2 had resilience, a portion of the peripheral surface of heating and fixing roll 2 which is brought into engagement with the peripheral surface of pressing roll 3 would be slightly covered as shown in FIG. 2, and copy sheet 1 would be discharged in the direction of an arrow d after its image has been fixed. Thus, copy sheets can be fed smoothly without being jammed.
The heating and fixing roll is preferably heat conductive for the following reason: It is only the peripheral surface of a heating and fixing roll that is required to be heated. Generally, heating rolls are constructed such that a heater serving as a heat source is built in the interior of each roll and surrounded by a layer of an insulating material, such for example as silicone rubber having a specific resistance of over 10.sup.15 .OMEGA.cm. Thus, conventional heating and fixing rolls have poor heat conductivity and the heat generating source must have a high wattage. In addition, the layer surrounding the heater must meet the requirement that it should be heat resistant.
Since conventional heating rolls each have a heater in the central portion, a portion of the rolls near the center axis tends to have an elevated temperature. Thus, when the heating rolls are made of rubber or other materials of low rigidity, they are softened by heat and become unfit to be used as heating rolls. This makes it necessary to use rubber of high rigidity to impart high mechanical strength to the heating rolls. However, it is not desirable to increase surface hardness of heating rolls in order to preclude the offset phenomenon.